1. Field of the Invention
The present invention relates to a demodulator for receiving and demodulating a pulse position modulated (PPM) signal transmitted in a radio communication system using an ultra wide band (UWB).
2. Description of the Background Art
In recent years, proposals have been made for radio communication systems using an ultra wide band. For example, as disclosed in U.S. patent application publication No. US 2003/0174779 A1, a UWB communication system spreads a data signal spectrum over an ultra wide band in the order of, for example, 2-6 GHz to accomplish pulse transmission.
In a conventional radio transmission system, a transmitter multiplies a carrier frequency by a modulation component to generate an output signal to be transmitted, while a receiver multiplies the received input signal by the same carrier frequency as that of the transmitter side to thereby extract the modulation component. The receiver was configured to include a mixer for multiplying a received signal by a frequency equal to the carrier frequency to convert it to a baseband signal, a processor for sampling the signal with high resolution to determine the signal, and a rake receiver for absorbing the delay time involved in waves arriving over multiple paths to thereby accomplish demodulation.
Also, there was a pulse transmission device for receiving pulse signals rather than such carrier signals, disclosed for example in Japanese patent laid-open publication No. 284129/1993. For example, in a pulse transmission system, such as a pulse position modulation (PPM) system, a waveform to be transmitted is pulsated and carries modulation information at a time position at which the waveform takes its significant level, thereby making it possible to maintain the transmission power as well as to make effective use of the transmission bandwidth.
However, the conventional radio transmission system, when applied to a UWB radio communication system, would cause the conventional configuration including a mixer for modulation and demodulation, a sampling circuit for sampling signals with high resolution and a rake receiver for determining demodulation to raise a problem that the scale of the circuits increases as the band is broadened.
Thus, in Japanese patent application No. 2004-179745 filed in the name of the same assignee as the present patent application, there is proposed a pulse modulator and a PPM demodulation determining circuit in ultra wide band radio communications. The PPM demodulation determining circuit is so configured that an output pulse transmitted is regarded as an electric power to then detect the absolute value of the received signal, integrate the latter for every integral time, and compare the resultant integrated values for the respective integral times with each other to determine at which position the pulse signal exists, thereby determining demodulation. That eliminates the need for the configuration of a mixer, a sampling circuit, a rake receiver and the like described above.
Now, in the specification of the above-mentioned patent application, there is disclosed the configuration for integration for every integral time. However, in order to attain the more optimal reception property, it has become necessary to seek a configuration that determines the length of integral time. Specifically, a PPM signal demodulator is called for, which is capable of optimizing the processing to implement signal control so as to make the reception property favorable without requiring the conventional configuration of a mixer, a sampling circuit and a rake receiver.